Master Linux Performance: Deep Dive into top, free, and vmstat Commands

1. top command

top is a common Linux performance analysis tool that shows real‑time resource usage of processes, similar to Windows Task Manager.

Usage: top [options] Options:

-b batch mode

-c show full command line

-I ignore idle processes

-s secure mode

-S cumulative mode

-i<seconds> set delay

-u<user> show only processes of a user

-p<pid> monitor specific PID

-n<count> number of iterations

Example – display process information: top Sample output (truncated) shows system time, uptime, load average, task counts, CPU states, memory usage, swap usage and a list of processes with columns PID, USER, PR, NI, VIRT, RES, SHR, S, %CPU, %MEM, TIME+, COMMAND.

Explanation of the first five lines:

Line 1 – current time, uptime, number of users, load average (1, 5, 15 min).

Line 2 – task summary: total, running, sleeping, stopped, zombie.

Line 3 – CPU states: %us (user), %sy (system), %ni (nice), %id (idle), %wa (IO wait), %hi (hardware IRQ), %si (software IRQ), %st (steal).

Line 4 – memory: total, used, free, buffers.

Line 5 – swap: total, used, free, cached.

Additional tips:

Press 1 to toggle per‑CPU view.

Press b to highlight the running process.

Press x to toggle column highlighting.

Use Shift > or Shift < to change the sort column (e.g., to %MEM).

2. free command

free displays the amount of free and used physical memory, swap space and kernel buffers.

Usage: free [options] Options:

-b display in bytes

-k display in kilobytes (default)

-m display in megabytes

-g display in gigabytes

-o omit the buffer/cache line

-s<seconds> continuously monitor

-t show total line (memory + swap)

-V show version

Example – show memory usage: free Typical output includes rows “Mem:” and “Swap:” with columns total, used, free, shared, buffers, cached, and a “-/+ buffers/cache” line that reflects memory available to applications.

Key points:

“used” in the first “Mem:” line counts kernel buffers and cache; the “-/+ buffers/cache” line subtracts them to show actual used/free memory for applications.

Available memory for an application can be approximated as free + buffers + cached.

When swap usage rises, the kernel is moving pages to disk, which may indicate insufficient RAM.

3. vmstat command

vmstat (Virtual Memory Statistics) reports statistics about processes, memory, paging, block I/O, traps and CPU activity.

Usage examples:

vmstat

vmstat 5 5

vmstat -a 2 5

vmstat -f

vmstat -s

vmstat -d

Common options:

-a show active and inactive memory

-f display fork count since boot

-s display memory‑related statistics

-d display disk statistics

-m display slab cache information

-n suppress repeated header lines

-S<unit> set output unit (k, K, m, M)

-t show total memory line

-V show version

Output fields (first line of a typical run):

procs: r (running), b (blocked on I/O)

memory: swpd, free, buff, cache

swap: si (swap‑in), so (swap‑out)

io: bi (blocks in), bo (blocks out)

system: in (interrupts), cs (context switches)

cpu: us, sy, id, wa, st

Interpretation notes:

High r > 4 or id < 40 % indicates heavy CPU load.

Non‑zero si/so suggests memory pressure.

Non‑zero bi/bo with b > 3 points to I/O bottlenecks.

Additional examples: vmstat -f – shows total number of forks. vmstat -s – prints a list of memory statistics (total memory, used, free, buffers, cached, swap, page‑ins/outs, interrupts, context switches, etc.). vmstat -d – displays per‑device read/write statistics. vmstat -m – shows slab allocator caches.

Understanding these tools helps administrators monitor system health, detect CPU, memory or I/O saturation, and make informed tuning decisions.